Why are some stars brighter than others?

Stars vary in brightness because of their intrinsic luminosity and because of their distance from us. More massive stars are larger and hotter and thus brighter than less massive stars. For example, Deneb, the brightest star in the constellation Cygnus, is about 20 times more massive than the Sun and has a luminosity 196,000 times that of the Sun. Proxima Centauri, the nearest star to the Sun, is about seven times smaller than the Sun and has a luminosity about 600 times less than that of the Sun.

The varying distances of the stars also play a large role in their brightness. To choose an extreme example, the Sun and Alpha Centauri A are the same type of star and have roughly the same luminosity. However, because the Sun is about 278,000 times closer to Earth than Alpha Centauri A, it appears much brighter to us.